In the United States television programs are broadcast on assigned channels by means of amplitude-modulated radio frequency (RF) carriers. The frequency of each carrier is the frequency assigned by the Government to a respective channel. Television programs are broadcast in accordance with the rules and regulations established by the Federal Communications Commission (FCC) and must also comply with the technical standards set by the National Television Standards Committee (NTSC). Thus a number of stringent requirements, both legal and technical, must be met by commercial television broadcasters.
Practically all television programs at the present time are broadcast in color. Previously, when there was only black and white broadcasting it was not necessary to deal with additional technical complications resulting from adding color information to the video picture signals being broadcast. Picture information in a color TV broadcast is contained in both the "luminance" and the "chrominance" portions of the video signals. At any pixel location of a picture being displayed on a TV screen, the brightness of the image and its color (red, green, or blue) are simultaneously derived from the luminance and chrominance portions of the signal. This mode of operation is well known in the art and need not be discussed further herein.
Distortion of the luminance portion of a TV signal usually results also in improper color reproduction of an image when it is subsequently displayed on a TV screen. The human eye is very sensitive to false renditions of colors, improper bleeding of one color into another, color streaks, etc. It is therefore highly desirable, in order to assure consistently high quality picture reproduction, that the luminance and chrominance portions of a color TV signal have their phases and amplitudes within the specifications set by NTSC standards and FCC regulations.
There are over 10,000 cable television (CATV) systems in operation in the United States, each distributing TV programs to subscribers via a respective cable network. Each network, by its nature, is local or geographically limited in extent. This however provides an important opportunity for local TV coverage of town or school events, for commercial advertisements of local goods or services, etc. It is usual therefore for a CATV operator to insert into regular programs, which are generally intended for viewers nationwide, programs of a local nature (ads, etc.) intended for viewing only by the subscribers of each particular CATV system. Frequently, these local programs are produced on a low budget, often using equipment and/or recording tapes which are of relatively low quality. Sometimes a locally produced program has video signal levels, which if inserted as-is into a regular program being broadcast over the CATV system would over-modulate the RF carrier of that respective TV channel. Over-modulation can cause severe distortion in TV pictures subsequently displayed on cable subscribers' TV sets. Excess white level peaks in a line video signal can also interfere with the proper functioning of TV picture scrambling, descrambling systems used by CATV operators. CATV operators have in the past "clipped" or electronically limited excess amplitude levels of the video signals at baseband before modulation of the RF carrier to limit distortion. Clipping was usually done with a conventional shunt diode circuit which selectively "clips" the signals above a certain level and thus limits amplitude of the video signal before it is used to modulate a carrier. However, as will be explained in detail hereinafter, the use of such conventional "clipping" circuits distorts the lower luminance portion of the video signal. This distortion in turn reacts with the chrominance portion of the signal to cause false colors, bleeding, streaking, etc., in the TV pictures when reproduced on a standard TV receiver.
In an effort to minimize the problems caused by "clipping", such as described above, the FCC recently issued Regulation #76,605 (a)(12) titled "Differential Gain" and Regulation #76,605 (a)(13) titled "Differential Phase". These regulations state: "As of Jun. 30, 1995, the following requirements apply to the performance of the cable television system as measured at the output of the modulating or processing equipment (generally the head end) of the system". The regulations then give definitions of "differential gain" and "differential phase", spell out specification limits not to be exceeded by these values, and give detailed instructions on how to measure differential gain and phase for the purposes of the regulations. These regulations are readily available from the FCC in Washington, D.C., and will not be discussed further. An article entitled "Differential gain, differential phase, chrominance-to-luminance delay", by Rex Bullinger, pages 58-70 in the September 1994 issue of Communications Technology also discusses testing and other technical matters relating to these FCC regulations.
If prior "clipper" circuits continue to be used, some CATV operators may be broadcasting signals that are out-of-compliance with the new FCC regulations. The reasons for this will be explained in detail hereinafter.
It is desirable to provide clipping circuitry and method, which meets the Jun. 30, 1995 distortion requirements of the FCC.